std lib iterators unnecessarily require value copies

[arnetheduck]

Description

When iterating over the pairs of a Table or (or indeed any other std lib type with a custom iterator), there's no fundamental reason why a copy of the keys/values should be needed and indeed, it is significantly expensive to require one for anything but non-trivial table entries.

Apart from being broadly inefficient, it also makes it hard to use move-only types in general because they can no longer be used with std containers.

import std/tables
type NoCopies = object

proc `=copy`(a: var NoCopies, b: NoCopies) {.error.}

var t: Table[int, NoCopies]
t[3] = NoCopies() # only moves
for k,v in t.pairs(): # lent values, no need to copy!
  echo "found entry"

Nim Version

2.2

Current Output

Error: '=copy' is not available for type <NoCopies>; requires a copy because it's not the last read of 't.data[h].val'; routine: main

Expected Output

Known Workarounds

No response

Additional Information

No response